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Aviation History
1946
1946 - 1076.PDF
554 FLIGHT MAY 30TH, 1945 AIR CONDITIONING LARGE CIVIL AIRCRAFT must be large and its cooling drag therefore excessive. If the efficiency of the expander is too low, the air may come out hotter than it went in. One scheme now under development follows the trend of modern power plants. A centrifugal blower compresses the air to a ratio of about 3:1, the heated air then passing through an air-cooled intercooler, finally expanding through a single-stage turbine. The mechanism thus resembles the gas turbine except that heat is ex- tracted at constant pressure instead of being added. The compression ratio being proportional to the square of the speed, difficulty might be experienced when driving from a main piston engine unless a constant-speed drive were em- ployed. With turbine main engines the difference between take-off and cruising r.p.m. is so small that fixed drive could be used. At first sight this installation could be combined with the main compressor if the latter were of cen- trifugal type; however, in this case it would not be possible to slow down the main blower to J speed at ground level, because there would then be no compression for the refrigerating system. It would be necessary to run at full speed, which would result in compressing and re- frigerating three or four times as much air as necessary. This would demand a huge intercooler (already quite large enough from the drag point of view) and a serious con- sumption of power, needing a substantial driving gear. One solution of this difficulty is to have a separate refrigerating machine running at constant speed and just large enough to supply all the air required at ground level. In addition, a main centrifugal blower with a three-or-four- to-one variable speed gear would gradually take up the supply as the altitude increased. Fig. 1 shows the power required to operate the refrigerator (100-passenger air- craft) if designed, as mentioned in the previous paragraph, to be just large enough for ground-level requirements. The upper curve shows the power absorbed by the com- pressor ; the lower curve the power regained in the turbine ; and the middle curve gives the net power required to drive the installation. A refrigerator is seldom necessary at high altitude, and the power shown can be saved by de-clutch- ing the drive. * Temperature Sequence Temperature drop through the refrigerating system depends upon the humidity, being greatest with dry air. The intercooler is supplied with atmospheric air through a suitable duct. Under dry tropical conditions the air may enter the machine at 104 deg. F, rise to 284 deg. F, drop to 140 deg. F in the intercooler, and finally fall in the expander to 32 deg. F. This is, of course, too low, and the intercooler shutters would be partially closed by a thermostat at the humidifier exit. However, with 50 per cent atmospheric humidity, the final temperature at admission to the cabin would be 68 deg. F. At higher initial humidities and temperatures the initial cabin tem- peratures will be somewhat higher than the ideal figure, but probably acceptable, especially soon after entering and before leaving the aircraft. The intercooler load for a typical 100-passenger aircraft is shown in Fig. 2. The intercooler may be 12m x 33m x i8in and weigh about 62 lb. With its shutters wide open it will require about 180 lb/min of cooling air at a pres- sure of 12m of water. The same duct will do for the inter- cooler and for the main compressor cooler, as they are not normally in action together. Detail sketch of Lysholm-type blower rotors showing complementary profiles of lobes for progressive compression. In very cold weather, near the ground, the same con- dition of dryness may obtain as at altitude. The air must, therefore, be heated before it reaches the humidifier, and near the ground the refrigerator will do this automatically. The humidifier thermostat would close the intercooler shutters and the mass-flow valve would by-pass the main compressor, thus the entering air would be compressed d re-expanded in the refrigerator without intercooling. Despite the high efficiencies of the installation, there would be enough-; < residual work done to supply \\/(l heat required. ff For all except the largest aircraft, and particularly for those having main compressors of the Roots type operating at 25,000ft or less, an alternative to the high-speed tur- bine refrigerator is to use two dis- ' placement compressors (mounted on the same gear box as the blower) with the usual intercooler between them. At the necessary pressure ratio of about 3.5:1 a Roots type blower has too low an adiabatic efficiency. Instead a compressor and expander of the Lysholm type might be used. While superficially resembling the Roots type, Lysholm blowers have rotors of peculiar form which draw in air at one end, progressively compress it within the rotors and deliver it continuously at the other end. The adiabatic efficiency is very high and, in the smaller sizes, is superior to that of centrifugal types. Combining Efficiencies As the compression ratio is determined by the geometry and not by the speed of rotation, this type is particularly suitable for use with piston engines, and a fixed drive may be used with some simple clutch to disengage the drive as required. The main blower, if of the Roots type, requires a fixed drive, and the three units (main blower, Lysholm compressor and Lysholm expander) could all be mounted on the same gearbox. For a 100-passenger aircraft the total power required for driving each (duplicated) set of main and refrigerating compressors is shown in Fig. 3 from which it will be seen that 80 h.p. is required at take-off, rising steadily during climb to a maximum of no h.p. at 12,000ft. If the simplicity of operation at low altitude of the Roots blower could be combined with the efficiency at high compression ratios of the Lysholm blower, the main compressor could be of this type up to 40,000ft, and a variable-speed drive would be unnecessary. This may be done by providing a Lysholm blower with a valve on its delivery side which, when open, virtually transforms the unit into a Roots blower with no internal compression ratio. Up to about 20,000ft the valve would be open>*- above this height the valve would be closed to obtain effi- cient compression at high altitudes. FORTHCOMING EVENTS May 30th.—R.Ae.S. The 34th Wilbur Wright Lecture by Mr. E. L Relf (I.C.E., Gt. George St.). June '1st.—R.Ae.S. Reading and Dist. Branch ; Aero- nautical Garden Party, Woodley. June 15th.—Old Cranwellian Assoc. reunion, R.A.F. College. June 15th.—No. 150 Sqn. R.A.F. reunion dinner, Dorchester Hotel, 6.30 p.m. June 19th.—Sir Frederick Handley Page at Gauge and Tool Makers' Assoc. luncheon, Savoy, 12.30 p.m. June 22nd.—Air Pageant, Southampton Airport. June 30th.—Northern Heights M.F.C. Gala day, Langley Airfield Nr. Slough.
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